As makeeasycash stated you first need to get certified. After that get yourself insurance, you can do so online in case a client gets injured. Once you've covered these two bases, listen to your client's needs and wants. Give advice but never press what YOU want to do for your client. Personal fitness is about setting goals that are attainable for your client. Some will require baby steps and lots of encouragement.
Learn to motivate without badgering, learn patience when clients whimper, and most of all, learn to listen, really listen to your client.